Effect of Pulsating Flow on Surge Frequency of a Turbocharger Compressor

Turbocharger compressors operate under pulsating flow due to continuous opening and closing of engine valves placed downstream of the compressor but exhibit a hysteresis loop deviating from steady characteristics. As the flow in a compressor is throttled, both pressure and flow discharge unstably oscillate. The instability is known as surge and it affects not only the compressor itself but also its whole engine-turbocharger system. For estimating the stable operation range, partial flow tests of a turbocharger compressor under pulsating flow were conducted. Using a compressor test apparatus with a pulsating flow generator and a tank whose capacity can be changed, the flow rate was controlled by a valve. Under steady flow, surge hysteresis loops caused by oscillating pressure and flow rate were obtained in each rotational speed. By Fast Fourier Transform (FFT) of pressure signals, the occurrence of surge with different frequencies was confirmed depending on the tank capacity. Next in pulsating flow tests, a compressor rotational speed is fixed, and pulsating frequencies given by the pulsating flow generator are changed. The obtained surge frequencies were different from those in steady flow tests. Especially, it is likely to be drawn to the frequency that is equal, two times and a half rate of the pulsating frequency. Numerical computation with one-dimensional calculation is conducted to investigate such a phenomenon.